The Kawasaki Ki-61 Hien ("Tony") & Ki-100

v1.2.0 / 01 jul 03 / greg goebel / public domain

* The Kawasaki Ki-61 Hien ("Swallow") fighter represented a major departure
for Japanese aircraft design in World War II. While other Japanese fighters
were designed with air-cooled radials and were optimized for maneuverability,
the Ki-61 used a liquid-cooled in-line engine and was designed for speed and
power.

In fact, the Ki-61 was so different from other Japanese fighters that when
the type was first encountered in combat over New Guinea in June 1943, the
Allies thought it wasn't a Japanese design at all. At first they believed it
was a copy of the German Messerschmitt Me-109, then suspected it was a copy
of the Italian Macchi C.202 Foglore or similar Italian fighter. For this
reason they gave it the code-name "Antonio", or "Tony", though by the summer
of 1943 the Allies had realized the Ki-61 was in fact a Japanese design.

The Hien proved initially successful in combat against American fighters. As
the war in the Pacific ground on, however, the Ki-61 found itself
increasingly outclassed, but it soldiered on until the end of hostilities.
This document describes the history of the Ki-61 and its radial-engine
derivative, the Ki-100.

* The confusion of the Ki-61 with German and Italian fighters had some basis
in the aircraft's origins. Between 1923 and 1933, Kawasaki Aircraft
Engineering Company's head designer was a German named Dr. Richard Vogt, who
returned to Germany in 1933 to take a similar position at the firm of Blohm
und Voss during the war. Not surprisingly, Kawasaki continued to be strongly
influenced by Dr. Vogt's beliefs after he left, particularly a faith in the
usefulness of liquid-cooled inline engines. This made Kawasaki something of
a heretic among Japanese aircraft manufacturers, who preferred air-cooled
radials.

In March 1938, Kawasaki signed an agreement with Daimler-Benz of Germany for
manufacturing rights to the liquid-cooled inline engines then under
development by the German firm. In April 1940, a Kawasaki engineering team
visited Daimler-Benz in Stuttgart to obtain plans and samples of the DB-601A
engine, then being used in the Me-109.

The Kawasaki engine team managed to increase the take-off power of their
version of the engine to 875 kW (1,175 HP) and reduce its weight slightly.
The engine was put into production in November 1941. It was designated the
"Ha-40", or "Army Type 2", though it would be later redesignated the "Ha-60"
in a combined Army/Navy nomenclature.

In the meantime, certain officers at the Air Headquarters of the Imperial
Japanese Army were very interested in the new fighters powered by
liquid-cooled being developed in Britain, the USA, the USSR, Germany, and
France. The Japanese Army also had unpleasant experiences in air combat
against the Soviet Polikarpov I-16 fighter during the beating the Imperial
Army took in their Manchurian border clash with the USSR in the summer of
1939. This experience suggested the the single-minded focus on agility above
all that characterized Japanese fighter design doctrine might need to yield
to a focus on speed and improved armor protection and firepower.

* In February 1940, the Army initiated work with Kawasaki on two single-seat
fighters based on the DB-601 derivative engine: a heavy interceptor,
designated the "Ki-60", and a general-purpose fighter, designated the
"Ki-61". Kawasaki decided to build the Ki-60 first, and the design team,
under Kawasaki chief designer Takeo Doi and his deputy Shin Owada,
constructed three prototypes of the interceptor in 1941.

The Ha-40 engine was not available at that time, so the three aircraft were
powered by sample DB-601A engines obtained from Germany. The Ki-60 was a
low-wing monoplane, with plenty of power and heavy armament by Japanese
standards. The first and second prototypes had two 12.7 millimeter (0.50
caliber) Ho-103 machine guns mounted on the nose in front of the pilot and
two 20 millimeter Mauser MG-151 cannons, one mounted in each wing, for a
total of four guns. The third prototype had four 12.7 millimeter guns.

Flight tests began in March 1941 and showed that the Ki-60 was too
unmaneuverable and didn't meet its speed and climb requirements. Various
tweaks to improve the aircraft failed, and so the Ki-60 was abandoned.

* The experience was valuable, however. Design work on the Ki-61, whose
development had been proceeding in parallel with the Ki-60 since December
1940, incorporated new features derived from the lessons learned by the Ki-60
program:

Aerodynamic refinements were added.

The wing was increased in size and length to improve maneuverability.

The fuselage was slimmed down to improve speed.

Armament was reduced by replacing the two 20 millimeter guns in the wings
with either two 12.7 millimeter Ho-103 guns or two Type 89 7.7 millimeter
(0.303 caliber) guns.

Fuel capacity was increased, as required by offensive fighter operations,
which dictated longer range than required by an interceptor.

The landing gear track was widened to allow use from primitive forward
airfields.

The first prototype was rolled out in early December 1941, and its
performance proved excellent. Eleven more prototypes were delivered to the
Army, which performed intensive trials with them. The Ki-61 was pitted
against other Japanese fighters, as well as against the Messerschmitt
Bf-109E-3, of which two had been bought by the Army from the Germans, and the
Curtiss P-40E, several of which had been captured during the seizure of the
Dutch East Indies.

While the test pilots were a little skeptical of the new aircraft at first,
pilots with combat experience appreciated the Ki-61's self-sealing fuel
tanks, heavier armor and armament, and fast diving speed. The air combat
tests showed the Ki-61 to be faster than all its adversaries in the tests,
and easily out-maneuvered all of them except the Japanese Nakajima Ki-43
("Oscar").

The 13th Ki-61, a production prototype, was delivered in August 1942. The
Army gave the green light for production, and the fighter began to roll off
the assembly line, with 34 delivered by the end of 1942. The type was
formally known as the "Army Type 3 Fighter Model 1 Hien", or "Ki-61-I".

The production machines differed from the prototypes only in tweaky details.
Initial production consisted of two variants: the "Ki-61-Ia", with two 12.7
millimeter guns in the nose and a 7.7 millimeter gun in each wings for a
total of four guns; and the "Ki-61-Ib", with 12.7 millimeter guns in both
fuselage and wings. These aircraft could be fitted with two 200 liter (53 US
gallon) drop tanks.

* The Hien entered combat in the spring of 1943 in the New Guinea war zone,
covering New Guinea, the Admiralty Islands, New Britain, and New Ireland.
The new Japanese fighter caused some pain and consternation among Allied
pilots, particularly when they found out the hard way that they could no
longer go into a dive and escape as they had from lighter Japanese fighters.
5th Air Force Commander General George Kenney found his P-40 Warhawks
completely outclassed, and begged for more P-38 Lightnings to counter the
threat of the new enemy fighter.

The Ki-61 demonstrated only a few teething problems in field use, such as a
tendency towards engine overheating during ground running under tropical
conditions. However, despite the heavier armament, it still didn't have the
punch to easily knock rugged and well-armed Allied bombers out of the sky.

The Kawasaki designers had forseen this problem. The Japanese Ho-5 20
millimeter cannon wasn't available at the time, but the Japanese obtained 800
Mauser MG-151/20 20 millimeter cannon from Germany in August 1943, and
modified 388 Ki-61-I airframes on the production line to carry the German
weapons in place of the two 12.7 millimeter wing guns. The cannon had to be
mounted on their sides to fit into a wing, with an underwing blister for the
breech, and some reinforcements were added to the wing to absorb the
increased recoil.

* Once the Ho-5 cannon became available, Kawasaki designers then reversed the
arrangement of the guns, putting the 20 millimeter cannon in the nose and the
12.7 millimeter guns in the wings. While they were making these
modifications, they also made a few changes to streamline manufacturing and
simplify field maintenance.

This new variant was designated the "Ki-61-I KAIc" (where "KAI" was for
"kaizo", or "modified") was 19 centimeters (7.5 inches) longer than its
predecessors, with a detachable rear section, a fixed tailwheel instead of
the retractable tailwheel previously used, stronger wings, and stores pylons
outboard of the main landing gear, allowing it to carry two 250 kilogram (550
pound) bombs.

The Ki-61-I KAIc went into production in January 1944 and ultimately replaced
production of all earlier models in August 1944. A few "Ki-61-I KAId" bomber
interceptors were also built in late 1944. These aircraft featured two 12.7
millimeter guns in the fuselage and a 30 millimeter Ho-105 cannon in each
wing.

Total production of all subvariants of the Ki-61-I was 2,654, with the
Ki-61-I KAIc accounting for over half that number.

* Even before the Hien saw combat, the Army had been pressing Kawasaki for an
improved version of the same aircraft. To this end, Kawasaki engineers
focused on an improved version of the Ha-40 engine known as the "Ha-140",
which was expected to have a take-off power of 1,120 kW (1,500 HP).

The first prototype of the new variant, the "Ki-61-II", flew in August 1943.
Ten more prototypes were ordered. They featured a wing with 10% more area,
and an improved canopy to provide better rear visibility, but the Ha-140
development program ran into troubles, and only eight of the prototypes
received engines. Even then, they suffered from engine troubles, structural
failures due to weaknesses in the new wing, and handling problems.

In an attempt to fix the problems, after delivery of the eighth Ki-61-II
prototype, the ninth prototype was extensively modified during manufacture.
The extended wing was replaced with the original Hien wing, the fuselage was
lengthened, and the rudder area increased. The result was the "Ki-61-II
KAI", with initial flight of the prototype in April 1944. It was followed by
30 more prototypes. As long as the temperamental Ha-140 engine worked
properly, the Ki-61-II KAI proved to be a promising interceptor, with a fast
climb rate and good high-altitude flight characteristics.

Despite the problems with the engine, the military situation was increasingly
desperate, and so the Ki-61-II KAI was put into production anyway in
September 1944. Two versions were produced, one designated "Ki-61-II KAIa",
with 12.7 millimeter guns in the wings and 20 millimeter cannon in the nose,
and the other designated "Ki-61-II KAIb", with four 20 millimeter cannon.

374 Ki-61-II KAI airframes were built and 99 of them fitted with engines.
Then, on 19 January 1945, US Army Air Force B-29 Superfortresses turned the
plant at Akashi that was building the Ha-140 engine into cinders and rubble.
That abruptly ended concerns over the reliability of the Ha-140 engine, but
left 275 airframes sitting around without powerplants.

* However, in November 1944 concerns over the availability (or lack thereof)
of the Ha-140 engine had led the Munitions Ministry to request that Kawasaki
redesign the Ki-61-II KAI to use an available engine. Company engineers
performed a lightning design effort to mate the fighter to the 1,120 kW
(1,500 HP) Mitsubishi Ha-112-II 14-cylinder double-row air-cooled radial
engine. The engineers inspected the radial engine installation of a sample
Focke-Wulf FW-190 fighter obtained from Germany and, in an example of
interservice cooperation that was far more the exception than the rule
between the Imperial Japanese Army and Navy, took advantage of Navy efforts
to use the Ha-112-II.

The first prototype conversion of three took to the air on 1 February 1945.
Sometimes improvisations work poorly, sometimes they work surprisingly well,
and the new variant demonstrated excellent performance. The rest of the
engineless Ki-61-II KAI airframes were then converted to the new fighter
type, which was designated the "Ki-100-Ia". They retained the armament of
the Ki-61-II KAIb, consisting of 12.7 millimeter guns in the wings and 20
millimeter guns in the fuselage.

Performance was roughly the same but engine reliability was vastly improved.
The Ki-100 was in fact an excellent fighter, a nasty customer for Allied
aircraft to deal with while being surprisingly comfortable and easy to fly,
an important consideration when experienced Japanese pilots were in
increasingly short supply.

A batch of 118 new-production Ki-100s were then built, incorporating an
all-round vision canopy, tested on a modified Ki-61-II KAI, and given the
designation "Ki-100-Ib".

Work was then done to add a turbosupercharger and water-methanol engine boost
to the Ki-100 to provide improved high-altitude performance. The first
prototype of this variant, designated the "Ki-100-II", flew in May 1945, with
two more prototypes completed before Japan's surrender ended plans for
production.

* As Allied forces pushed in the bounds of Japan's overextended ocean empire,
the Hien fought in the South Pacific, in the Philippines, on Okinawa, and
finally in defense of the Japanese home islands themselves. Some Hien units
also served on in China and on Formosa.

The home defense units operating on Japan used a mixed bag of Ki-61 variants,
essentially anything they could get their hands on. The most famous of these
units was the 244th Sentai (Fighter Group) under Japanese ace Major Tembico
Kobayashi. Major Kobayashi encouraged his men to perform frontal attacks on
B-29s and press their assaults to close range, even ramming if that was what
was needed. Major Kobayashi set an example by fighting with almost suicidal
determination.

By this time, the Hien was outclassed by American Mustangs and other
late-model US fighters. In fact, when attacking B-29s Hiens often had to be
protected by Ki-100s. Operations dwindled as planes and pilots continued to
fall in combat and fuel and spare parts dried up. By the end, the home
defense units were no longer capable of posing a real threat to the
Americans.

Of the 3,159 Hiens built, not many survived the war. Several were evaluated
by the Americans and one was presented back to Japan by the US Air Force
later. This was the only surviving Hien as of the mid-1960s. American
aircraft restorer Kermit Weeks has a Ki-61 airframe in slow process of
restoration.

* The Ki-61 wasn't the only fighter design to be powered by the Ha-40 engine.
Kawasaki also experimented with an unusual twin-engine design conceived by
Takeo Doi in 1939 and approved for prototype development in October 1940 as
the Kawasaki "Ki-64".

The Ki-64 actually had both engines in a line, straddling the cockpit and
driving a single contra-rotating prop system with twin three-bladed props.
The combined engine was known as the "Ha-201", and generated 1,755 kW (2,350
HP) take-off power. It featured an unusual steam cooling system with
radiators in the wing and flap surfaces. The front engine used the right
wing for cooling, while the rear engine used the left wing. The Ki-64 had a
vague configurational resemblance to the Hien, though it was larger.

One prototype was finished in December 1943, but the rear engine caught fire
on the fifth test flight. The aircraft made an emergency landing and
survived. However, it was never repaired and the project was abandoned in
the middle of 1944.

* I seem to be strongly inclined in my writing to describe aircraft I made
models of when I was a kid. This is something of an attempt at vindication,
I think, because I was such an inept modelmaker. However, I do recall that I
did a fair-to-middling job on my 1/72 scale Ki-61, and it still sticks in my
mind, though the sleek "sky shark" look of the Hien helped make it memorable
as well.

One of the puzzles in writing about Japanese aircraft is trying to figure out
whether to use the Japanese name ("Hien") or the Allied codename ("Tony") in
the text. Obviously one of the reasons for giving codenames was to give
Allied forces a more easily recognized "handle" for the aircraft than the
Japanese designation, if the Allies even knew what the Japanese designation
was when a new aircraft appeared.

Using the Allied codename might be a bit more convenient, but somehow giving
an aircraft a name assigned by an enemy seems disrespectful. The Hien is the
hero of its own story and even if it was used against my own countrymen, it
deserves to be respected on its own terms.